The present invention refers to a method of producing a ferroelectric memory, a method of storing information on a substrate and to a memory device.
The storage of information is becoming increasingly important with the advent of larger datasets. Computer programs and algorithms have become more complex and larger, and therefore the demand for an optimization of storage space has grown, aiming at a miniaturization of the individual unit in which the information is to be stored. Conventional storage methods rely for example on optically or magnetically readable/writeable media which, however, have certain limitations with respect to the minimum size possible.
Electronic computers have grown more powerful as their basic sub-unit, the transistor, has shrunk during the past forty years. However, limitations imposed by quantum mechanics and fabrication techniques are likely to inhibit further reduction in the minimum size of today""s bulk-effect semiconductor transistors. It is projected that conventional devices may not function well as the overall size of the semiconductor transistor is aggressively miniaturized to below approximately 0.1 micrometers (100 nm). In order to continue the miniaturization of the circuit elements down to the nanometer scale, perhaps even to the molecular scale, researchers are investigating several alternatives to the transistor. These new nanometer-scale electronic (nanoelectronic) devices perform both as switches and amplifiers just like today""s transistors. However, unlike today""s transistors, which operate based on the movement of masses of electrons in bulk matter, the new devices take advantage of quantum mechanical phenomena that emerge on the nanometer scale, including the discreteness of electrons.
A group of materials that have been found to be potentially useful for information storage are ferroelectric materials. Just like with ferromagnetic materials, used for example in conventional audio tapes, ferroelectric materials display a hysteresis behavior with regards to their electrical polarization under the influence of an external electrical field.
Ferroelectric materials are a subgroup of noncentrosymmetric crystalline materials that have a spontaneous electrical polarization, the direction of which can be altered by an external electrical field. The polarization P versus applied electrical field E dependency shows hysteresis behavior, with P having two stable remanent values (+Pr and xe2x88x92Pr) when E=0. The (reverse) electrical fields that must be applied to annihilate the existing polarization (P=0) are termed the coercive fields (+Ec and xe2x88x92Ec). The term xe2x80x9cferroelectricxe2x80x9d was coined because the P-E relation of these materials is very similar to the B-H relation of ferromagnetic materials. This hysteresis behavior is the basis of the use of both kinds of materials in memory devices. Ferroelectrics are also analogous to ferromagnets in that they are characterized by a Curie temperature Tc (above which they become paraelectrical) and that they have an internal domain structure.
The hysteresis loop, shown in FIG. 1, is caused by the existence of permanent electrical dipoles. The curve starts at the origin (P=O) when the material is first produced because the ferroelectric domains are randomly oriented. When the external field is applied, the B4+ ions become displaced in the direction of the field, and domains that are more favorably aligned with the field grow at the expense of those that are not. This procession results in a rapid and major polarizing effect until a saturation level Ps (=saturation polarization) is reached, when the polarization vector of most of the domains are aligned with the field (dashed curve in FIG. 1). Removal of the field at this point eliminates any normal ionic polarization, but the B4+ ions remain in their field-oriented sites, and a remanent polarization +Pr is observed at E=O. In order to remove this polarization, it is necessary to apply an opposing field to revert half of the domains to favor the new field direction. That condition occurs when the opposing field reaches the material-specific coercive field xe2x88x92Ec. Continuation of the field cycle inverts the polarization to another saturation level, and removal of the negative field leaves the remanent polarization xe2x88x92Pr. Further cycles of the electrical field retrace the original path, creating a continuous hysteresis loop. The initial condition of P=O when E=O can only be again achieved by short-circuiting the capacitor and subjecting it to a temperature above Tc to generate a new system of random ferroelectric domains.
Some inorganic (ceramic) ferroelectric materials have the perovskite structure, i.e. ABO3, with A=a large divalent metal cation and B=a small tetravalent metal cation. Examples include BaTiO3, PbTiO3 and PbZrxTi1xe2x88x92xO3 (PZT). The structure consists of 12 coordinated A2+ ions on the comers of a cube, octahedral O2xe2x88x92 ions on the faces, and tetrahedral B4+ ions in the center (see for example FIG. 2). The Curie temperature in these materials is associated with a structural transition from regular cubic above Tc to a distorted tetragonal form below Tc.
The hysteresis loop typical of ferroelectrical materials is the basis for ferroelectric random access memory (FRAM) devices. As ferroelectric materials possess two stable polarization directions at zero field, they can be used as non-volatile memory elements. The direction of polarization is used to store information, a logical xe2x80x9c0xe2x80x9d corresponding to one direction and a logical xe2x80x9c1xe2x80x9d corresponding to the other direction. Generally, the device structure used in FRAM cells is either the ferroelectric field-effect transistor (FEFET) or the ferroelectric capacitor (FECAP). Computer memory devices utilizing the electrooptical properties of ferroelectric materials have also been described [Munroe, M. R.; Snaper, A. A.; Gregory, G. D. (1972) U.S. Pat. No. 3,675,220: xe2x80x9cPlanar random access ferroelectric computer memory.xe2x80x9d and Ogdfen, T. R.; Gookin, D. M. (1988) U.S. Pat. No. 4,731,754: xe2x80x9cErasable optical memory material from a ferroelectric polymer.xe2x80x9d].
Ferroelectric memory devices are generally susceptible to three forms of degradation in performance during use:
Electrical fatigue, which is defined as a decrease in the magnitude of the switchable polarization with increasing number of switching cycles.
Imprint failure, which is a polarization-driven field-shift of the hysteresis loop.
Aging, which is an ill-defined term generally indicating a degradation of the ferroelectric properties (capacitance and dielectric loss) with time.
These phenomena apparently derive from relaxation processes occurring at crystal boundaries. Fatigue and imprint are related to charge trapping at domain boundaries.
Recent work at the experimental level has shown that nanometer-scale polarization domains can be created in inorganic ferroelectric thin films using a conductive atomic force microscope (AFM). Domains as small as 30 nm diameter can be formed in a ferroelectric organic thin film on a conductive substrate by applying electrical pulses with an Au-conductive AFM tip. Binary information can be xe2x80x9cwrittenxe2x80x9d by changing the polarity of the applied electrical pulses and xe2x80x9creadxe2x80x9d by using piezoelectrical measurements [Matsushige, K., Yamada, H.; Tanaka, H.; Horiuchi, T.; Chen, X. Q. (1998) Nanotechnology 9,208-211: xe2x80x9cNanoscale control and detection of electric dipoles in organic molecules.xe2x80x9d]
The miniaturization of electronic components by using particle technology on the nanoscale is a way to circumvent some of the physical limits and expense of conventional methods of fabrication [Goldhaber-Gordon, D.; Montemerlo, M. S.; Love, J. C.; Opiteck, G. J.; Ellenbogen, J. C. (1997) Overview of Nanoelectronic Devices; The MITRE Corporation (http;//www.mitre.org/technology/nanotech), and Ellenbogen, J. C. (1998) A Brief Overview of Nanoelectronic Devices; The MITRE Corporation http://www.mitre.org/technology/nanotech).].
EP 0 788 149 A1 describes a method of depositing nanometer scale particles on a substrate in which Au particles are coated with negatively charged citrate ions, and the substrate is treated with a positively charged surfactant such that the opposite charges on the substrate and the Au particles are attracted to each other, and a thin film of particles is thus deposited. This structure is used as a tunnelling-gate in a transistor-like arrangement. Additionally in this document, a memory device is considered, based on the storage of information in the form of an excess electron. One bit of information according to the invention of this document corresponds to the presence or absence of one electron on the individual particle.
There are various ways to synthesize crystalline particles of ferroelectric materials on the nanoscale such as BaTiO3 and PbTiO3, which are known in the art. Surface modification of these particles by adsorption or covalent attachment makes it possible to change their interfacial properties without changing the intrinsic properties of the core material. Surface modification is useful for dispersing particles, immobilizing them onto surfaces, embedding them in organic films, etc. Such possibilities have already been demonstrated with ferroelectric particles. PZT particles 70 nm in diameter were silylated, spread as monoparticulate layers on water surfaces, and transferred to conductive glass substrates [Kotov, N. A.; Zavala, G.; Fendler, J. H. (1995) J. Phys. Chem. 99, 12375-12378: xe2x80x9cLangmuir-Blodgett films prepared from ferroelectric lead zirconium titanate particles.xe2x80x9d]. These layers exhibited ferroelectric properties, as was evidenced by polarization measurements [Kotov, N. A. et al. (1995) ibid.]
Nevertheless, there are problems with these techniques: the ferroelectric properties of inorganic thin films (xe2x89xa610 nm) tend to disappear due to some defects of the structures and/or composition at the interface, while the polarization domains written into the organic thin films are not stable temporally [Matsushige, K., Yamada, H.; Tanaka, H.; Horiuchi, T.; Chen, X. Q. (1998) Nanotechnology 9, 208-211: xe2x80x9cNanoscale control and detection of electric dipoles in organic molecules.xe2x80x9d]
Also, today""s ferroelectric memory cells rely essentially on bulk-effect behavior for their function. These devices are susceptible to degradation in performance by various relaxation processes occurring at the crystal boundaries. Although significant steps have been made toward write/read ferroelectric memory capabilities below 100 nm, the approaches so far have not shown much promise for development into practical memory elements.
Accordingly there exists a need in the art for an efficient method of storing information based on ferroelectric devices which do not exhibit the degradation and/or instability behavior hitherto encountered with conventional ferroelectric devices.
The object of the present invention is to provide a method of storing information based on ferroelectric particles, which method allows a miniaturization of memory devices down to the nanometer scale whilst at the same time avoiding instability and degradation behavior commonly encountered with ferroelectric devices.
The object is solved by a method of producing a ferroelectric memory which method comprises:
a) providing ferroelectric particles 10, 10xe2x80x2,
b) providing a substrate 30,
c) orientating at least a subset of said ferroelectric particles such that there is an axis of said particles along which axis a dipole moment may be directed in the ferroelectric state, said axis having an orientation the average of which is in at least one predetermined direction with regard to a surface of said substrate,
d) allowing said ferroelectric particles to attach to said substrate.
In one embodiment, the order of steps c) and d) may be reversed. In one embodiment, the order of steps a) and b) may be reversed.
It is also envisaged that said axis may have an orientation the average of which is in two or more non-identical predetermined directions with regard to a surface of said substrate. xe2x80x9cThe average of whichxe2x80x9d is meant to designate a state in which, within an ensemble of particles, the average orientation of these particles within that ensemble is the predetermined direction with regard to a surface of said substrate.
In one embodiment said at least one predetermined direction is essentially perpendicular to a surface of said substrate.
In another embodiment said at least one predetermined direction is at an oblique angle to a surface of said substrate. xe2x80x9cObliquexe2x80x9d in this context is meant to signify any value x taken from the range 0xc2x0 less than x less than 180xc2x0. Preferably x is taken from the range 45xc2x0xe2x89xa6xxe2x89xa6135xc2x0.
The object is also solved by a method of producing a ferroelectric memory which method comprises:
a) providing ferroelectric particles 10, 10xe2x80x2,
b) providing a substrate 30,
c) orientating the electrical dipoles 20, 20xe2x80x2 of said ferroelectric particles such that the orientation of each dipole is essentially perpendicular to a surface of said substrate,
d) allowing said ferroelectric particles to attach to said substrate.
In one embodiment the order of steps a) and b) may be reversed. In one embodiment the order of steps c) and d) may be reversed.
It is preferred that said ferroelectric particles attach to said substrate in a manner that they are separated from each other on said substrate. This can, for example, be achieved by chemical moieties attached to the particles, the moieties carrying a net charge each, which will lead to a mutual repulsion between the particles.
In a preferred embodiment said ferroelectric particles attach to said substrate by electrostatic interactions.
In one embodiment, said orientating occurs by electrostatic interactions. This can, for example, be achieved by chemical moieties, carrying a net charge each, which are attached to said particles, which charges cause the particles to be orientated in a specific orientation.
It is preferred that said substrate exhibits charges of one electrical polarity, and wherein said ferroelectric particles exhibit charges of the opposite electrical polarity.
In one embodiment of the invention said charges exhibited by said substrate are generated by application of an electrical potential and optionally by adjusting a pH-value, and/or they are charged moieties appended to said substrate. The process of generating/changing charges on a substrate by applying an electrical potential in order to attract particles of opposite charge is referred to as xe2x80x9celectrophoretic depositionxe2x80x9d, e.g. hereafter in the example section.
In a preferred embodiment said charges exhibited by said ferroelectric particles are charged moieties appended to said particles.
It is preferred that said charged moieties are negative and comprise groups which can be represented by the general form XOxe2x88x92 or XSxe2x88x92, where X is covalently attached to Oxe2x88x92 or Sxe2x88x92 by one of its constituent atoms, said constituent atom being an atom selected from the second through sixteenth column and second through sixth row of the Periodic Table. Preferably XOxe2x88x92 is selected from the group comprising phenolate and carboxylate (a C-atom being the link to Oxe2x88x92), phosphate and phosphonate (a P-atom being the link to Oxe2x88x92), sulfate and sulfonate (an S-atom being the link to Oxe2x88x92), borate (a B-atom being the link to Oxe2x88x92), Arsenate and Arsenite (an As-atom being the link to Oxe2x88x92). Preferably XSxe2x88x92 is selected from the group comprising thiolate and dithiocarbamate (a C-atom being the link to Sxe2x88x92), dithiophosphate and dithiophosphonate (a P-atom being the link to Sxe2x88x92).
In one preferred embodiment said charged moieties are positive and comprise groups which can be represented by the general form C4P+ or C3S+, where C is a carbon atom having sp3 hybridisation, or by the general form CaNHb+, where C is a carbon atom having either sp3 or sp2 hybridisation and the sum of the coefficients a and b equals 3 in the case of sp3 hybridisation, or 2 in the case of sp2 hybridisation.
Positively charged moieties of the form C4P+ are known generally as xe2x80x9cphosphoniumxe2x80x9d ions and those of the form C3S+ are known generally as xe2x80x9csulfoniumxe2x80x9d ions. Another way to express these moieties is R(PR1R2R3)+ and R (SR1R2)+, respectively, where R is an alkyl or aryl residue used for the appendage, and R1, R2, and R3 are alkyl or aryl groups which may be the same or different and may be connected (cyclic). For the purpose of the present invention, the latter groups should not be very large, and suitable examples are methyl (CH3) and ethyl (CH2CH3) groups. It should also be noted that the alkyl or aryl groups can themselves be substituted. For example, 2-hydroxyethyl (CH2CH3OH) groups might be used instead of ethyl groups to enhance the polarity. Positively charged moieties of the form CaNHb+ are known generally as xe2x80x9cammonium ionsxe2x80x9d. The possibilities of these N-based moieties are greater because H can be used instead of alkyl or aryl groups, but there must be at least one alkyl or aryl R group for the appendage. Another way to express these moieties is RNH3+ (xe2x80x9cprimaryxe2x80x9d ammonium), RNR1H2+ (xe2x80x9csecondaryxe2x80x9d ammonium), RNR1R2H+ (xe2x80x9ctertiaryxe2x80x9d ammonium), and RNR1R2R3+ (xe2x80x9cquaternaryxe2x80x9d ammonium) ions. As with the phosphonium and sulfonium moieties, the R1, R2, and R3 groups may be the same or different and may be connected (cyclic). An important class of heterocyclic tertiary and quaternary ammonium moieties are those derived from pyridine (xe2x80x9cpyridiniumxe2x80x9d ions).
It is preferred that said charged moieties are appended via silylation.
It is also preferred that said charged moieties are appended via complexation. xe2x80x9cComplexationxe2x80x9d here refers to a coordination complex formed at the surface of the particle involving one or more metal atoms intrinsic to the particle and the donor atoms of a ligand.
In a preferred embodiment said charged moieties are metal complexes.
It is preferred that said charged moieties are appended to a polymer.
In principle, any charged polymer that can bind to the particle surface could be useful. Some groups of the polymer would interact with atoms on the particle surface to anchor it to the particle and the rest would provide charged moieties to interact with the solvent. In Example 6, see below, carboxylic acid groups of the poly(acrylic acid) serve both functions, binding to the barium titanate particle principally through the titanium atoms. There are numerous other kinds of polymers that could be used instead, including poly(phosphates), poly(amino acids), and poly(ethylene imine).
In a preferred embodiment said ferroelectric particles are single-domain particles.
It is preferred that said particles are 5-200 nm in size, preferably 10-150 nm, more preferably 20-100 nm in size, the size of each particle being determined by the longest dimension of said particle.
Preferably said ferroelectric particles are formed by compounds selected from the group comprising mixed oxides containing comer sharing oxygen octahedra.
In a preferred embodiment said ferroelectric particles are formed by compounds selected from the group comprising perovskite-type compounds.
It is preferred that said ferroelectric particles are formed by compounds selected from the group comprising tungsten-bronze type compounds.
Preferably said ferroelectric particles are formed by compounds selected from the group comprising bismuth oxide layer-structured compounds.
In a preferred embodiment said ferroelectric particles are formed by compounds selected from the group comprising lithium niobate or lithium tantalate.
In one embodiment said ferroelectric particles are formed by compounds selected from the group which can be represented by the general form AlBmOn, wherein
A is selected from the group comprising Li+, Na+, K+, Ca2+, Sr2+, Ba2+, La3+ (or other rare earth metal ion), Co3+, Cd2+, Pb2+, and Bi3+.
B is selected from the group comprising Mg2+, Ti4+, Zr4+, Hf4+, Nb5+, Ta5+, W5+, Mn3+, Fe3+, Ni2+, Zn2+, Al3+, Ga3+, Sn4+, and Sb5+, and
l, m, and n are integral values such that the sum of positive valences contributed by the atoms in groups A and B equals 2 n.
It is preferred that said ferroelectric particles form a layer on said substrate.
Preferably, said layer has a thickness in the range of 5-200 nm.
In one embodiment said layer has a thickness in the range of 10-150 nm.
It is preferred that said layer is a monolayer.
In one embodiment of the invention, said substrate forms part of an electrode, said electrode being at least partially immersed in a suspension of said particles.
It is preferred that said electrode forms part of an electrical cell, an electrical field being created in said electrical cell, said electrical field causing said substrate to exhibit charges of one polarity.
It is preferred that an adjustment of a pH-value of said suspension is used to cause said substrate to exhibit charges of one polarity.
It is also preferred that an adjustment of a pH-value of said suspension is used to cause said particles to exhibit charges of one polarity.
In a preferred embodiment, said pH-value of said suspension is adjusted to the range of 7-9.
It is preferred that, after attachment of said particles to said substrate, said particles are fixed to said substrate, preferably by means of drying and/or crosslinking and/or drying under vacuum.
As used herein, xe2x80x9cattachmentxe2x80x9d is meant to designate any contacting interaction. xe2x80x9cAttachmentxe2x80x9d can, for example, occur by electrostatic forces, but other forces are not intended to be excluded, as used herein. If an attachment by covalent forces is intended, it is herein referred to explicitly as xe2x80x9cattachment by covalent interactionsxe2x80x9d. The term xe2x80x9cbeing fixedxe2x80x9d, however, is meant to designate a contacting interaction which is primarily covalent in its nature.
In a preferred embodiment, said ferroelectric particles attach to said substrate by affinity interactions.
In a preferred embodiment, said ferroelectric particles attach to said substrate by covalent interactions.
The object is also solved by a method of storing information on a substrate wherein, in a device obtainable by a method of producing a ferroelectric memory according to the present invention, the electrical dipoles of said particles are directed by directing means 140.
The object is furthermore solved by a method of storing information on a substrate wherein, in a device obtainable by a method of producing a ferroelectric memory according to the present invention, a permanent dipole moment in the ferroelectric state is created in all or at least a plurality of said particles by directing means 140, resulting in groups of particles having essentially the same permanent dipole moment.
A xe2x80x9cgroup of particlesxe2x80x9d may be an array of e.g. 1xc3x972, 2xc3x972, 3xc3x973, 4xc3x974, 2xc3x973, 2xc3x974, 3xc3x974, etc. particles.
In one embodiment said dipole moment in the ferroelectric state is essentially perpendicular to a surface of said substrate.
In another embodiment said dipole moment in the ferroelectric state is essentially at an oblique angle to a surface of said substrate.
It is preferred that said directing means is selected from the group comprising probes of scanning probe microscopes (SPM) for applying electrical pulses.
In a preferred embodiment of the invention, said creating of said dipole moment occurs by applying electrical pulses to individual ferroelectric particles and/or by applying electrical pulses to groups of particles.
In a preferred embodiment of the invention, said directing of said electrical dipoles occurs by applying electrical pulses to individual ferroelectric particles and/or by applying electrical pulses to groups of particles.
It should be mentioned that the term xe2x80x9cdirecting said electrical dipolesxe2x80x9d is meant to designate, for example, an action by which it is determined whether an electrical dipole points xe2x80x9cupxe2x80x9d or xe2x80x9cdownxe2x80x9d along its axis of orientation. xe2x80x9cOrientating said ferroelectric particlesxe2x80x9d is meant to designate, for example, an action by which an axis of said particles is established along which a xe2x80x9cdirecting actionxe2x80x9d of dipoles can occur.
The object is also solved by a memory device comprising
a substrate, and
ferroelectric particles attached thereto, wherein the orientation of the electrical dipoles of said particles is, on average, at least one predetermined direction with regard to a surface of said substrate.
It is preferred that said at least one predetermined direction is an essentially perpendicular direction with regard to a surface of said substrate.
It should be noted that the term xe2x80x9cat least one predetermined directionxe2x80x9d is meant to enclose embodiments, in which there are two or more, non-identical predetermined directions.
In another embodiment said at least one predetermined direction is an oblique direction with regard to a surface of said substrate. xe2x80x9cOblique directionxe2x80x9d in this context means that it forms an angle with said surface of said substrate the value x of which is taken from the range 0xc2x0 less than x less than 180xc2x0. Preferably x is taken from the range 45xc2x0xe2x89xa6xxe2x89xa6135xc2x0.
It is preferred that said ferroelectric particles attach to said substrate in a manner that they are separated from each other on said substrate.
In a preferred embodiment of the invention, said ferroelectric particles attach to said substrate by electrostatic interactions.
It is preferred that said substrate exhibits charges of one electrical polarity, and wherein said ferroelectric particles exhibit charges of the opposite electrical polarity.
It is preferred that said charges exhibited by said substrate are generated by application of an electrical potential and/or by adjusting a pH-value, and/or they are charged moieties appended to said substrate.
It is preferred that said charges exhibited by said ferroelectric particles are charged moieties.
In one embodiment said charges exhibited by said ferroelectric particles are charged moieties appended to said particles.
It is preferred that said charged moieties are negative and comprise groups which can be represented by the general form XOxe2x88x92 or XSxe2x88x92, where X is covalently attached to Oxe2x88x92 or Sxe2x88x92 by one of its constituent atoms, said constituent atom being an atom selected from the second through sixteenth column and second through sixth row of the Periodic Table. Preferably XOxe2x88x92 is selected from the group comprising phenolate and carboxylate (a C-atom being the link to Oxe2x88x92), phosphate and phosphonate (a P-atom being the link to Oxe2x88x92), sulfate and sulfonate (an S-atom being the link to Oxe2x88x92), borate (a B-atom being the link to Oxe2x88x92), Arsenate and Arsenite (an As-atom being the link to Oxe2x88x92). Preferably XSxe2x88x92 is selected from the group comprising thiolate and dithiocarbamate (a C-atom being the link to Sxe2x88x92), dithiophosphate and dithiophosphonate (a P-atom being the link to Sxe2x88x92).
In one embodiment said charged moieties are positive and comprise groups which can be represented by the general form C4P+ or C3S+, where C is a carbon atom having sp3 hybridisation, or by the general form CaNHb+, where C is a carbon atom having either sp3 or sp2 hybridisation and the sum of the coefficients a and b equals 3 in the case of sp3 hybridisation, or 2 in the case of sp2 hybridisation.
In one embodiment said charged moieties are appended via silylation.
It is preferred that said charged moieties are appended via complexation. xe2x80x9cComplexationxe2x80x9d here refers to a coordination complex formed at the surface of the particle involving one or more metal atoms intrinsic to the particle and the donor atoms of a ligand.
In one embodiment said charged moieties are metal complexes.
Preferably said charged moieties are appended to a polymer.
In a preferred embodiment of the invention, said ferroelectric particles are single-domain particles.
It is preferred that said particles are 5-200 nm in size, preferably, 10-150 nm, more preferably 20-100 nm in size, the size of each particle being determined by the longest dimension of said particle.
In a preferred embodiment of the invention, said ferroelectric particles are formed by compound selected from the group comprising mixed oxides containing comer sharing oxygen octahedra.
Preferably said ferroelectric particles are formed by compounds selected from the group comprising perovskite-type compounds.
In a preferred embodiment said ferroelectric particles are formed by compounds selected from the group comprising tungsten-bronze type compounds.
Preferably said ferroelectric particles are formed by compounds selected from the group comprising bismuth oxide layer-structured compounds.
It is preferred that said ferroelectric particles are formed by compounds selected from the group comprising lithium niobate or lithium tantalate.
In one embodiment said ferroelectric particles are formed by compounds selected from the group formed which can be represented by the general form AlBmOn, wherein
A is selected from the group comprising Li+, Na+, K+, Ca2+, Sr2+, Ba2+, La3+ (or other rare earth metal ion), Co3+, Cd2+, Pb2+, and Bi3+.
B is selected from the group comprising Mg2+, Ti4+, Zr4+, Hf4+, Nb5+, Ta5+, W5+, Mn3+, Fe3+, Ni2+, Zn2+, Al3+, Ga3+, Sn4+, and Sb5+, and
l, m, and n are integral values such that the sum of positive valences contributed by the atoms in groups A and B equals 2 n.
It is preferred that said ferroelectric particles form a layer on said substrate.
Preferably, said layer has a thickness in the range of 5-200 nm.
Preferably, said layer is a monolayer.
The object is also solved by a use of a ferroelectric memory produced by the method of producing a ferroelectric memory according to the present invention, or by the use of a ferroelectric memory obtainable by the method of producing a ferroelectric memory according to the present invention, or by the use of a memory device according to the present invention for the storage of binary information, wherein, preferably, said ferroelectric memory or said memory device is used in an electronic component.
The object is also solved by a method of storing information on a substrate which method comprises:
providing ferroelectric particles,
providing a substrate,
providing the substrate with receptors for said ferroelectric particles,
providing said ferroelectric particles with ligands enabling said particles to bind to said receptors,
allowing said ferroelectric particles to bind to said substrate, whereby said ferroelectric particles form a monolayer, and
orientating the electrical dipoles of said ferroelectric particles by orientating means.
The object is also solved by a memory device comprising
a substrate, and
a monolayer of ferroelectric particles bound thereto.
The object is also solved by the use of a memory device manufactured by the method according to the present invention or of a memory device obtainable by the method according to the present invention or of a memory device according to the present invention, for the storage of binary information.
In one embodiment of the method according to the present invention, it is preferred that the binding of said ferroelectric particles to said substrate is governed by electrostatic interactions.
According to a preferred embodiment, said receptors provide charges of one electrical polarity to said substrate.
In one embodiment, it is preferred that said ligands provide charges of the opposite electrical polarity to said particles.
According to one preferred embodiment, said receptors are charges generated by application of an electrical field and/or by adjusting a pH-value.
It is preferred that said ligands are charged moieties affixed to silane derivatives, wherein it is especially preferred that said affixed moieties are selected from the group comprising 3-aminopropyl and N-(2-aminoethyl)-3-aminopropyl.
In a preferred embodiment, said ferroelectric particles are uniformly sized.
Preferably, said ferroelectric particles are single-domain particles.
It is preferred that said particles have essentially cubic and/or essentially spherical morphology.
In one embodiment said particles form plate-like crystals.
In one embodiment said particles are 5-200 nm in size, preferably 10-150 nm, more preferably 20-100 nm, this numerical value referring to either the diameter or the length of the longest edge of the particle.
In a preferred embodiment, said particles are crystalline.
It is also preferred that said ferroelectric particles are formed by inorganic ferroelectric compounds.
In a preferred embodiment, said ferroelectric particles are formed by compounds selected from the group comprising perovskite-type compounds, barium titanate, lead circonate, wherein, more preferred, the perovskite-type compounds are selected from the group comprising BaTiO3, PbTiO3, PbZrxTi1xe2x88x92xO3 (PZT), ABX3, wherein
A is selected from the group comprising Ca2+, Ba2+, Pb2+, Sr2+, Cd2+, K+, Na+, and rare earth metals,
B is selected from the group comprising Ti4+, Zr4+, Sn4+, Nb5+, Hf4+, Ta5+, W5+, and Ga3+, and
X is selected from the group comprising O, F, Cl, Br, and I.
In a preferred embodiment, said monolayer has a thickness in the range of 5-100 nm.
Preferably, said substrate is electrically conducting.
The invention may also provide that said substrate is smooth compared to the dimensions of said particles.
In one embodiment of the method according to the present invention, said substrate is formed by a substance selected from the group comprising glass, surface-modified glass, glass, that has an extra layer attached to its surface, glass with an extra layer of fluorine-doped tin oxide (Fxe2x80x94SnO2).
It is preferred that said substrate forms part of an electrode in an electrical cell, said electrical cell being capable of adopting an open-circuit condition (xe2x80x9copen-circuit electrical cellxe2x80x9d) or a closed-circuit condition (xe2x80x9cclosed-circuit electrical cellxe2x80x9d), wherein it is preferred that said electrode is at least partially immersed in a suspension of said particles, and wherein it is also preferred that said electrical cell is polarised by application of an external electrical field, said electrical cell now being a closed-circuit electrical cell.
In a preferred embodiment, a pH-value of said suspension is in the range of 2-6 in said open-circuit electrical cell, and/or a pH-value of said suspension is in the range of 7-9 in said closed-circuit electrical cell.
In a preferred embodiment, after the binding of said particles to said substrate, said monolayer is fixed to said substrate, wherein it is preferred that said monolayer is fixed to said substrate by means of drying and/or crosslinking and/or drying under vacuum.
In a preferred embodiment, said orientating means is selected from the group comprising probes of scanning probe microscopes (SPM) for applying electrical pulses.
It is preferred that said orientating of the electrical dipoles occurs by applying electrical pulses to individual ferroelectric particles.
In one embodiment of the invention, the binding of said ferroelectric particles to said substrate is governed by affinity interactions.
In another embodiment of the invention, the binding of said ferroelectric particles to said substrate is governed by covalent interactions.
In a preferred embodiment of the memory device according to the invention, said ferroelectric particles are bound to said substrate by interactions of receptors on said substrate with ligands on said ferroelectric particles.
It is preferred that said ferroelectric particles are bound to said substrate by electrostatic interactions.
In a preferred embodiment, said receptors provide charges of one electrical polarity to said substrate, and/or said ligands provide charges of the opposite electrical polarity to said particles.
It is preferred that said receptors are charges generated by application of an electrical field and/or by adjusting a pH-value.
It is preferred, that said ligands are selected from the group comprising (3-aminopropyl)triethoxysilane and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane.
In a preferred embodiment, said ferroelectric particles are uniformly sized.
It is preferred that said ferroelectric particles are single-domain particles.
The invention may also provide that said particles have essentially cubic and/or essentially spherical morphology.
In a preferred embodiment said particles are 5-200 nm in size, preferably 10-150 nm, more preferably 20-100 nm, the size being determined by the longest dimension of the particle. This longest dimension can, for example, be the diameter or the length of the longest edge of the particle.
In a preferred embodiment, said particles are crystalline.
In a preferred embodiment, said ferroelectric particles are formed by inorganic ferroelectric compounds.
It is also preferred that said ferroelectric particles are formed by compounds selected from the group comprising perovskite-type compounds, potassium sodium tartrate, barium titanate, lead circonate wherein, more preferred, the perovskite-type compounds are selected from the group comprising BaTiO3, PbTiO3, PbZrxTi1xe2x88x92xO3 (PZT), ABX3, wherein
A is selected from the group comprising Ca2+, Ba2+, Pb2+, Sr2+, Cd2+, K+, Na+, and rare earth metals,
B is selected from the group comprising Ti4+, Zr4+, Sn4+, Nb5+, Hf4+, Ta5+, W5+, and Ga3+, and
X is selected from the group comprising O, F, Cl, Br, and I.
In a preferred embodiment, said monolayer has a thickness in the range of 5-100 nm.
Preferably, said substrate is electrically conducting.
The invention may also provide that said substrate is smooth compared to the dimensions of said particles.
In a preferred embodiment of the memory device according to the present invention, said substrate is formed by a substance selected from the group comprising glass, surface-modified glass, glass, that has an extra layer attached to its surface, glass with an extra layer of fluorine-doped tin oxide (Fxe2x80x94SnO2).
In a preferred embodiment of the use according to the present invention, the memory device is used in an electronic component.
xe2x80x9cFerroelectric particlesxe2x80x9d are particles exhibiting ferroelectric properties. The term xe2x80x9csubstratexe2x80x9d refers to any entity to which particles can attach. The term xe2x80x9cmonolayerxe2x80x9d of substance A refers to a film comprising a single layer of molecules/atoms of substance A. As used herein the term xe2x80x9cthe binding of . . . is governed by electrostatic interactionsxe2x80x9d refers to a relation in which the binding is accompanied by and/or due to electrostatic interactions. The terms xe2x80x9creceptorxe2x80x9d and xe2x80x9cligandxe2x80x9d refer to two entities capable of interacting with and binding to each other. They can be functional groups, groups with opposite electrical polarity, but also merely charges of opposite electrical polarity induced by the application of an electrical field or generated by other means. The term xe2x80x9cparticles are uniformly sizedxe2x80x9d means that the particles, on average, have essentially the same size. The term xe2x80x9csingle-domain particlesxe2x80x9d is meant to designate the fact that such a particle is a single ferroelectric domain. A ferroelectric domain is an entity in which all electrical dipoles are aligned resulting in a net electrical polarisation. The term xe2x80x9celectrically conductingxe2x80x9d refers to a capability of transporting charges, electrons, electron-holes, ions, etc. The term xe2x80x9csaid substrate is smooth compared to the dimensions of said particlesxe2x80x9d is meant to designate a state in which the roughness of the substrate measured by the mean dimension of its peaks (and the enclosed valleys) is smaller than the mean dimensions of the particles. The terms xe2x80x9copen-circuit conditionxe2x80x9d and xe2x80x9cclosed-circuit conditionxe2x80x9d designate an electrical circuit which is open and closed respectively. The term xe2x80x9csaid electrode is at least partially immersedxe2x80x9d designates a state in which all or part of the electrode""s surface is immersed.
The present invention uses arrays of ferroelectric particles, preferably as individually addressable, non-volatile memory elements. In one embodiment, the particles of choice exhibit a narrow size distribution and are highly crystalline (nanocrystals), single-domain ferroelectric substances. According to the present invention, one bit of information corresponds to the xe2x80x9cupxe2x80x9d- or xe2x80x9cdownxe2x80x9d-direction of the electrical polarization of one individual particle. Alternatively, the memory unit cell can be made up of small arrays (e.g. 1xc3x972, 2xc3x972, 3xc3x973, 2xc3x973, 2xc3x974, 4xc3x974 . . . etc.) or clusters of particles instead of single particles.
The present invention provides the following advantages compared to the state of the art:
It enables the miniaturization of non-volatile ferroelectric memory elements down to the 100 nm scale and below.
The memory elements are not subject to degradation via mechanisms present in polycrystalline devices because the ferroelectric domains are isolated from one another as individual particles.
The readout method is non-destructive.
No state of the art memory device, based on ferroelectric or other aforementioned behaviour, provides all of these features.